I. Field of the Invention
The present invention relates to a method of forming both a resist pattern and the resist processing apparatus used in this method.
II. Description of the Prior Art
As the integration density of semiconductor devices increases, as in super LSIs (large scaled integrated circuits), high precision micropattern formation techniques become necessary. The dimensional requirements of a pattern are very exacting. In the most advanced fields, a dimensional precision of 3.sigma..ltoreq.0.1 .mu.m-.sigma. indicates deviation from an average pattern size- is now necessary for 5- or 6-inch masks. In order to utilize pattern formation techniques in a mass production line, size deviations between photomasks or wafers must be suppressed to fall within the range of 3.sigma..ltoreq.0.15 .mu.m.
Meanwhile, in order to effectively mass produce pattern formations, a resist, having not only a high sensitivity but also a sensitivity suitable for the exposure apparatus used, is required.
Conventionally, the following method for forming a resist pattern has been adopted for use. A resist is coated on a substrate (e.g., a mask substrate) by a spin coating or dipping method. The resist film on the substrate is baked to a predetermined temperature (Tb) by a heating means (e.g., an oven or hot plate). After baking for a predetermined period of time, the resist film-coated substrate is allowed to cool naturally in atmosphere at ambient temperature and pressure for 20 to 30 minutes. After cooling, the resist film on the substrate is exposed with a suitable dose of radiation. The resist film on the substrate is then developed and rinsed, thus forming a resist pattern.
In the conventional method, however, it is difficult to obtain a resist film having uniform sensitivity. Sensitivity not only varies from one portion to another in a single surface of a resist film, but furthermore, variations in sensitivity also occur between resist films on individual substrates. Consequently, even if exposure conditions are constant, it is very difficult to form a resist pattern of high precision either on a surface of a single substrate or the surfaces of a plurality of substrates.
The present inventors undertook extensive studies to determine the cause of the variations in the size of resist patterns formed by the conventional method. It was found that the sensitivity of a resist becomes nonuniform due to uneven cooling after the baking process, resulting in variations in the size of the resist pattern. It was confirmed that when a substrate with a resist film is naturally cooled while leaving it to stand vertically after the baking process, the upper portion of the resist film is cooled in accordance with curve A in FIG. 1, and its lower portion is cooled in accordance with curve B in FIG. 1. When the sensitivity of the naturally cooled resist film is checked, the upper and lower portions of the resist film exhibit the sensitivity characteristics indicated in FIG. 2 by curves A' and B', respectively. Note that FIG. 2 shows a residual rate of film thickness due to a change in exposure dose. As can be understood from a comparison of FIGS. 1 and 2, a strong correlation was found between the cooling rate and the sensitivity of a resist film, and this was determined as being the cause of variations in the size of resist patterns.
Because the conventional resist film formation method employs a cooling step during which the cooling rate is uncontrolled, the sensitivity of the formed resist film becomes unstable and nonuniform, thus hindering the formation of a precisely dimensioned resist pattern.
As a means of responding to the need for a highly sensitive and uniformly dimensioned resist, it is conceivable that a method in which, after baking, a substrate is conveyed into a chamber where it can cool under conditions of precise temperature control, may be proposed. A method such as this, however, would, in addition to being complex, be very expensive.
In view of the above situation, a method for increasing the cooling rate of a resist film after baking so as to obtain a high-sensitivity resist, for example, a method of dipping a substrate in a fluid in which resist does not dissolve, has been proposed. However, a drying step made necessary by the dipping step is complex. In addition, although high sensitivity is achieved, variations in sensitivity in a single substrate surface are undesirably enhanced to a greater extent than when the conventional method, employing natural cooling, is used. Therefore, with these methods, it is difficult to form a highly sensitive and uniformly dimensioned resist pattern without employing a complex and expensive method.